Getting Warmer? Prehistoric Climate Can Help Forecast Future Changes

Using fossil animals, plants, and pollen, U.S. Geological Survey (USGS) scientists and collaborators around the world have reconstructed climate conditions that existed on the Earth during a period 3.3-3.0 million years ago, known as the mid-Pliocene warm period. This global reconstruction is the first of its kind for a period as warm as that predicted for the end of the 21st century. It will improve understanding of today's warming and, more importantly, assist testing and refinement of climate models that estimate future warming.

Above: Microfossils used in mid-Pliocene paleoclimate research. Clockwise from top left: ostracode, planktic foraminifer, diatom, benthic foraminifer, and pollen grain. One way scientists reconstruct past climate is by comparing the spatial distribution of these microfossils with the distribution of their relatives living in today's climate. Calcareous microfossils (foraminifera and ostracoda) carry additional climate information in their elemental ratios; for example, the ratio of magnesium to calcium varies with ocean-water temperature, and the ratios of different isotopes of oxygen vary with global ice volume and, therefore, sea level. [larger version]

"PRISM's research provides data for climate modelers to better understand the environment in which we live, and for decision makers to develop informed adaptation and mitigation strategies that yield the greatest benefits to society and the environment," said Senior Advisor to USGS Global Change Programs Thomas Armstrong. "This is one of the most comprehensive global reconstructions for any warm period and emphasizes the importance of examining the past state of Earth's climate system to understand the future."

Past warm periods provide valuable data on climate change and serve as natural laboratories for understanding the global climate system. The mid-Pliocene warm period is particularly useful because—in contrast to earlier warm periods, such as the Late Cretaceous (approx 100-65 million years ago)—the mid-Pliocene was a time when many conditions on Earth were similar to those of today: the continents were in virtually the same positions, and most of the plant and animal species alive then are still extant. Temperatures, however, were considerably higher: global average temperatures were 2.5°C (4.5°F) greater than today, placing them within the range projected for the end of the 21st century by the Intergovernmental Panel on Climate Change (IPCC).

"Exploring the mid-Pliocene will further our understanding of the role of ocean circulation in a warming world, the impacts of altered storm tracks, polar versus tropical sensitivity, and the impacts of changes in atmospheric CO2 and oceanic energy-transport systems," said USGS scientist Harry Dowsett, lead scientist for PRISM. "We used fossils dated to the mid-Pliocene to reconstruct sea-surface and deep-water ocean temperatures, and will continue research by studying specific geographic areas, vegetation, sea-ice extent, and other environmental conditions that existed during the Pliocene."

Above: Average February sea-surface temperatures during the mid-Pliocene, derived from the PRISM3_SST_v1.0 dataset. Sea-surface temperature is key to paleoclimate work because it is the temperature at the interface between air and sea—where sea ice forms (which affects Earth’s albedo, or surface reflectivity), where cold upwelling cells or warm surface currents affect the climate of nearby coastal zones, and where wind interacts with the water surface to create such phenomena as El Niño. Sea-surface-temperature maps show scientists how well or how poorly ocean currents distribute heat from the tropics to the poles. [larger version]

Above: Mid-Pliocene versus modern sea-surface-temperature anomaly for the month of February. Yellow, orange, and red colors indicate areas where average sea-surface temperatures were warmer during the mid-Pliocene than today. Note warm temperatures along the west coast of South America, a pattern similar to modern El Niños, and warm temperatures in the high latitudes of the North Atlantic and Arctic Oceans. [larger version]

Because CO2 levels during the mid-Pliocene were only slightly higher than today's, PRISM research suggests that a slight increase in our current CO2 level could have a large impact on temperature change. Research also shows warming of as much as 18°C in the high latitudes of the North Atlantic and Arctic Oceans during the mid-Pliocene, raising average temperatures there from -2°C to 16°C. Also characteristic of the mid-Pliocene was warming in the eastern Pacific, similar to a present-day El Niño (a periodic change in atmospheric circulation that affects ocean temperatures and weather patterns; see "What is El Niño?"). Global sea-surface and deep-water temperatures were determined to be warmer than those of today, affecting the ocean's circulation system and climate. Data suggest that the likely cause of mid-Pliocene warmth was a combination of several factors, including increased heat transport from equatorial regions to the poles and increased greenhouse gases.

The synthesis of paleoenvironmental data—such as PRISM's new reconstruction for the mid-Pliocene warm period—and the modeling of Earth's climate have been evolving in tandem. Paleoenvironmental studies provide new data that climate modelers can use to test their models, and unexpected model results commonly lead paleoclimatologists to important new research topics. This mutually beneficial relationship continues to expand; for example, the mid-Pliocene reconstruction by PRISM has been chosen by the Paleoclimate Modelling Intercomparison Project Phase II as the dataset against which to run and test the performance of climate models for the Pliocene.

The use of PRISM data to refine climate models and reduce the uncertainty in future climate projections is the most important impact of the research for Dowsett, who said: "Climate change is happening; it's something our children are going to have to deal with in a very real way. Knowing that what we're doing is making a difference is extremely rewarding."

Marci Robinson, who joined the USGS as a Mendenhall Postdoctoral Fellow to work with Dowsett on the PRISM project, agreed and added, "I also want to note how exciting it's been to bring together scientists from so many disciplines to work on a problem on such a global scale."

Robinson is lead author of a report about the new reconstruction, titled "Pliocene Role in Assessing Future Climate Impacts," published in the December 2, 2008, issue of Eos (v. 89, no. 49, p. 501-502). Her coauthors are Dowsett and Mark A. Chandler, of the National Aeronautics and Space Administration (NASA) Goddard Institute for Space Studies at Columbia University. Columbia is one of PRISM's primary collaborators; others are Duke University, the University of Leeds, and the British Antarctic Survey.

To hear a podcast interview featuring Dowsett and Robinson discussing the new climate reconstruction, listen to episode 77 of USGS CoreCast. For additional information and to view the compiled data, visit the PRISM3D Project.